1. Field of the Invention
The present invention relates to the illumination unit and more particularly to the illumination unit for projection systems.
2. Description of Related Art
The projector system requires a light source to illuminate the imager for projection. The light emitting diode (LED) is a preferable choice because of its small physical size, low-power consumption, long lifetime and availability of three primary colors. The LED has been widely used in many areas of general lighting, and driven by these applications to make it better and better in optical efficiency and color saturation.
The LED is typically packaged with an LED die or dies on a metal support for heat dissipation. The electrical connections are made from the LED die or dies to the bonding pads, which are on top of the metal support by a layer of isolation. Most of the LED die or dies are further surrounded and protected by a plastic encapsulation. The typical plastic encapsulation is a flat layer or a hemisphere dome of epoxy. This plastic encapsulation can extract more light from the LED die or dies, and is referred as the primary optics of the LED. The LED in the flat encapsulation emits the light in a well-known Lambertian distribution, and the LED in the hemisphere encapsulation emits the light in a more collimated Gaussian distribution.
FIG. 1 shows a conventional LED 10, which includes a support 11, a LED die or array of dies 12, bonding pads 13 and a plastic encapsulation 14. The support 11 is shown as a flat metal support, on which the LED die or dies 12 are adhered to. The bonding pads 13 were on top of the metal support with a layer of isolation 15, and the electric connections 16 are made form the LED die or dies 12 to the bonding pads 13. A plastic encapsulation 14 in a form of flat layer, hemisphere or any other curvature surrounds the LED die or dies 12 for the protection. This plastic encapsulation 14 is also served as the primary optics to extract more light from the LED die or dies 12 with different angular distributions.
FIG. 2 shows a diagram of the projection system with the LED. In detail, it shows a typical reflective-type projector system 20 with an LED 21 as the illuminant. The LED 21 can be the same as the LED 10 shown in FIG. 1. A secondary optics 22 is placed in front of the LED 21 to converge the light from a wider angular distribution to a more directional angular distribution. The light will then pass through an optional pre-polarizer 23 to a polarizing beam splitter (PBS) 24, which reflects the polarized light to a reflective imager 25. The typical reflective imager is for example a liquid-crystal-on-silicon (LCOS) microdisplay. This reflective imager 25 modulates and reflects the light back to the PBS 24, and then through an optional post-polarizer 26 to a projection lens 27 for the projection.
FIG. 3 shows a conventional secondary optics 30 of the LED used in the projector systems. The LED 31 can be the same as the LED 10 shown in FIG. 1. The secondary optics 30 is a set of lenses, which can be spherical, aspherical, or diffractive lenses. This set of lens elements is commonly referred as the condenser lens. The LED 31 is placed in the focal point of the condenser lens. The light emission from the LED and falls into this condenser lens can be collected by this condenser lens and be converged in a more directional and collimated way for illumination of the imager in the projector system. Only the light, which enters the condenser lens, can be collected by the condenser lens. Therefore, the diameter of the condenser lens has to be large and the focal length has to be shorter for the condenser lens to collect more light from the LED emission. As a result, the condenser lens by the spherical or aspherical construction is large and bulky. It is also difficult to collect all the light from the LED 41, and hence, the coupling efficiency of the LED to the imager is low. Replacing the spherical or aspherical lens by a diffractive lens can reduce the thickness of the condenser lens, but the coupling efficiency is still low and further suffered by the scattering loss on the diffractive surface of the diffractive lens.
FIG. 4 shows another secondary optics 40 of the LED 41 for light illumination. The LED 41 can be the same as the LED 10 shown in FIG. 1. The secondary optics 40 is a compound parabolic collimator (CPC) and is placed above the LED 41. The light emission from the LED 41 enters the flat bottom surface 42 of the CPC and will be reflected at the parabolic wall 43 by total internal reflection (TIR), and then exits from the flat top surface 44. This secondary optics 40 can collect the light emission which is from the LED 41 and enters the bottom surface 42, and pass the light out through the top surface 44 in a specific angular distribution according to the parabolic wall 43. This kind of secondary optics is a long and symmetrical cylinder, and passes the light into a symmetric cone for particular light illumination.
FIG. 5 shows another secondary optics 50 of the LED 51 for general lighting. The LED 51 can be the same as the LED 10 shown in FIG. 1. The secondary optics 50 is placed on top of the LED 51 and covers the LED 51. The secondary optics with an inner refractive lens 53, an inner refractive wall 54, an outer reflective wall 55 and a flat top surface 56, is placed on top of the LED 51 and covers the LED 51. This secondary optics 50 can collect all the light emission from the LED 51, and pass the light out in a specific angular distribution according to the surfaces of 53, 54, 55 and 56. The surfaces 55 and 56 are typical straight or flat. A subsequent patent (U.S. Pat. No. 6,547,423) proposed to change these two surfaces to more generally cured surfaces for an improved performance and reduced size. This kind of secondary optics is a shallow and symmetric disk, and passes the light into a symmetric cone for general lighting.
In summary, the primary optics of the LED is to extract more light from the LED die or dies to increase the external efficiency of the LED, while the secondary optics will be more application dependent. For the projector system, the secondary optics of the LED is to improve the directionality of the light emission from the LED to the imager of the projector system, or to increase the coupling efficiency of the LED to the imager of the projector system with all the light extracted from the primary optics of the LED. However, the light emission from the LED is not collimated or directional enough for the projector system. In addition, the light emission from the LED is in a rotational symmetric cone, which does not match the aspect ratio of the imager of the projector system such as 4:3 or 16:9. For the necessity, a new structure of the illumination unit is preferred for the projector systems.